Ultrashort laser pulse wafer scribing

ABSTRACT

Systems and methods are provided for scribing wafers with short laser pulses so as to reduce the ablation threshold of target material. In a stack of material layers, a minimum laser ablation threshold based on laser pulse width is determined for each of the layers. The highest of the minimum laser ablation thresholds is selected and a beam of one or more laser pulses is generated having a fluence in a range between the selected laser ablation threshold and approximately ten times the selected laser ablation threshold. In one embodiment, a laser pulse width in a range of approximately 0.1 picosecond to approximately 1000 picoseconds is used. In addition, or in other embodiments, a high pulse repetition frequency is selected to increase the scribing speed. In one embodiment, the pulse repetition frequency is in a range between approximately 100 kHz and approximately 100 MHz.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/440,792, filed May 25, 2006, which is hereby incorporated byreference herein in its entirety.

TECHNICAL FIELD

This application relates to laser cutting or scribing and, inparticular, to a method of manufacturing integrated circuitry usingultrashort laser pulses at a high repetition rate to ablate material.

BACKGROUND INFORMATION

Integrated circuits (ICs) are generally fabricated in an array on or ina semiconductor substrate. ICs generally include several layers formedover the substrate. One or more of the layers may be removed alongscribing lanes or streets using a mechanical saw or a laser. Afterscribing, the substrate may be throughcut, sometimes called diced, usinga saw or laser to separate the circuit components from one another.

Semiconductor manufacturers have been shrinking transistor sizes in ICsto improve chip performance. This has resulted in increased speed anddevice density. To facilitate further improvements, semiconductormanufacturers use materials to reduce the capacitance of dielectriclayers. For example, to form a finer circuit pattern, a semiconductorwafer having a low dielectric constant (low-k) insulating film islaminated on the surface of the semiconductor substrate. Low-kdielectrics may include, for example, an inorganic material such as SiOFor SiOB or an organic material such as polyimide-based or parylene-basedpolymer.

However, conventional mechanical and laser cutting methods are not wellsuited for scribing many advanced finished wafers with, for example,low-K dielectric materials. Relatively low density, lack of mechanicalstrength and sensitivity to thermal stress make low-k dielectricmaterial very sensitive to stress. Conventional mechanical wafer dicingand scribing techniques are known to cause chips, cracks and other typesof defects in low-k materials, thus damaging the IC devices. To reducethese problems, cutting speeds are reduced. However, this severelyreduces throughput.

Further, known laser techniques can produce excessive heat and debris.Traditionally, laser pulse widths in the tens of nanoseconds or morehave been used for semiconductor cutting or scribing. However, theselong pulse widths allow excessive heat diffusion that causes heataffected zones, recast oxide layers, excessive debris and otherproblems. For example, FIG. 1 is a side view schematic of asemiconductor material 100 diced using a conventional laser cuttingtechnique. Near a cut area 102, a heat affected zone 104 and recastoxide layer 106 has formed. Cracks may form in the heat affected zone104 and reduce the die break strength of the semiconductor material 100.Thus, reliability and yield are reduced. Further, debris 108 from thecut area 102 is scattered across the surface of the semiconductormaterial 100 and may, for example, contaminate bond pads.

In addition, conventional laser cutting profiles may suffer from trenchbackfill of laser ejected material. When the wafer thickness isincreased, this backfill becomes more severe and reduces dicing speed.Further, for some materials under many process conditions, the ejectedbackfill material may be more difficult to remove on subsequent passesthan the original target material. Thus, cuts of low quality are createdthat can damage IC devices and require additional cleaning and/or wideseparation of the devices on the substrate.

A method for laser cutting or scribing that increases throughput andimproves cut surface or kerf quality is, therefore, desirable.

SUMMARY OF THE DISCLOSURE

The embodiments disclosed herein provide systems and methods of scribinga finished wafer that includes low-k dielectric and/or other materialsas fast as or faster than existing mechanical and/or laser methods.However, the laser scribing is performed with reduced or no mechanicaland/or thermal stress and with reduced or no debris. Thus, little or nopost process cleaning is required. Further, clean, straight edge cutsare produced with no additional lateral separation of the devices onwafer required to accommodate the scribing process.

In one embodiment, a method of cutting a plurality of layers formed overa substrate is provided. Each of the plurality of layers has arespective laser ablation threshold that varies with laser pulse width.The method includes determining a minimum laser ablation threshold foreach of the plurality of layers and selecting the highest of the minimumlaser ablation thresholds. The method also includes generating a beam ofone or more laser pulses having a fluence in a range between theselected laser ablation threshold and approximately ten times theselected laser ablation threshold, and scribing a kerf between aplurality of integrated circuits formed in the plurality of layers. Thekerf passes through the plurality of layers to a top surface of thesubstrate.

In certain such embodiments, the laser pulses have a pulse width in arange between approximately 0.1 picosecond and approximately 1000picoseconds. Further, the beam has a pulse repetition rate in a rangebetween approximately 100 kHz and approximately 100 MHz and can cutthrough approximately 10 μm of material at a speed in a range betweenapproximately 200 mm/second and approximately 1000 mm/second. Inaddition, or in other embodiments, the energy per pulse is in a rangebetween approximately 1 μJ and approximately 100 μJ.

In another embodiment, a method is provided for scribing a wafer havinga plurality of integrated circuits formed thereon or therein. Theintegrated circuits are separated by one or more streets. The methodincludes generating a beam of one or more laser pulses. The laser pulseshave a pulse width selected so as to minimize an ablation threshold of atarget material. The method further includes ablating a portion of thetarget material with the beam at a pulse repetition frequency in a rangebetween approximately 5.1 MHz and approximately 100 MHz.

In another embodiment, a method is provided including generating a beamof one or more laser pulses that have a pulse width in a range betweenapproximately 0.6 picosecond and approximately 190 picoseconds. Themethod further includes ablating a portion of the target material withthe beam.

In another embodiment, a method is provided including generating a beamof one or more laser pulses that have a pulse width in a range betweenapproximately 210 picoseconds and approximately 1000 picoseconds. Themethod further includes ablating a portion of the target material withthe beam.

Additional aspects and advantages will be apparent from the followingdetailed description of preferred embodiments, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view schematic of a semiconductor material diced usinga conventional laser cutting technique.

FIGS. 2A-2C are side view schematics of an exemplary work piece that iscut according to certain embodiments of the invention.

FIG. 3A is a perspective view of a work piece cut according to anotherembodiment of the invention.

FIG. 3B is a side view schematic of the work piece shown in FIG. 3A.

FIG. 4 graphically illustrates the difference between a simplifiedGaussian beam irradiance profile and a simplified shaped beam irradianceprofile.

FIGS. 5A-5C graphically illustrate the difference between beam crosssection profiles.

FIGS. 6A-6D show exemplary substantially uniform irradiance profilesproduced by a Gaussian beam propagating through a diffractive opticalelement (DOE) according to certain embodiments of the invention.

FIG. 7 is an electron micrograph of a kerf scribed through interconnectand low-k dielectric layers according to an embodiment of the invention.

FIG. 8 schematically illustrates sequentially exposing a work piece tolaser pulses in the direction of a cut according to an embodiment of theinvention.

FIG. 9 is an electron micrograph of a micromachined pattern in asemiconductor material using laser ablation methods according to anembodiment of the invention.

FIG. 10 is an electron micrograph of a micromachined pattern in asemiconductor material using laser ablation according to an embodimentof the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The ability of a material to absorb laser energy determines the depth towhich that energy can perform ablation. Ablation depth is determined bythe absorption depth of the material and the heat of vaporization of thematerial. Parameters such as wavelength, pulse width duration, pulserepetition frequency, and beam quality can be controlled to improvecutting speed and the quality of the cut surface or kerf. In oneembodiment, one or more of these parameters are selected so as toprovide a substantially low fluence (typically measured in J/cm²) thathas just enough energy to ablate the target material. Thus, the amountof excessive energy deposited into the material is reduced oreliminated. Using a lower fluence reduces or eliminates recast oxidelayers, heat affected zones, chipping, cracking, and debris. Thus, diebreak strength is increased and the amount of post-laser cleaningrequired is decreased.

U.S. Pat. No. 5,656,186 to Mourou et al. teaches that the ablationthreshold of a material is a function of laser pulse width. As usedherein, “ablation threshold” is a broad term that includes its ordinaryand customary meaning, and includes, for example, a sufficient fluencerequired to remove material for scribing or cutting. Traditional pulsewidths in the nanosecond range generally require a higher ablationthreshold as compared to that of shorter pulse widths. Shorter pulsesincrease peak power and reduce thermal conduction. To increase spatialresolution, the Mourou et al. patent teaches using pulse widths in thefemtosecond range. However, femtosecond laser pulse widths removesmaller amounts of material per pulse as compared to traditionalnanosecond pulses. Thus, the amount of time required to cut or scribe aline is increased and throughput is reduced. Further, in the femtosecondpulse range, the ablation threshold may increase as the femtosecondpulses become shorter.

Thus, in one embodiment disclosed herein, pulse widths are selected inthe picosecond range to lower the ablation threshold while removing morematerial per pulse than femtosecond pulses. In the picosecond range, thetime constant for electrons initially excited by the laser pulse toexchange energy with the bulk of material (e.g., electron thermalizationwith consecutive electron-lattice interaction) is in the picosecondrange. For example, the time constant may be on the order ofapproximately 1 to 10 picoseconds. Thus, it is thought that pulses ofshorter or comparable duration result in “cold” Coulomb type ablationwithout significant heating. Accordingly, thermal stressing and/ormelting of the material is eliminated or reduced.

Artisans will recognize from the disclosure herein that pulses in therange between approximately 1 picosecond and approximately 10picoseconds may provide some thermal type ablation. However, usingrelatively low fluence per pulse that is only slightly above theablation threshold reduces excessive energy that produces melted debris.Thus, cleaner kerfs are produced. Further, the heat effects aregenerally limited to the laser spot because the pulse widths are tooshort for heat to diffuse or propagate outside the irradiated area.However, when the pulse becomes too short, the effective depth of thelaser light interaction with the material is shortened and theefficiency of the ablation is reduced (e.g., less electrons areinitially excited by the laser pulse).

To increase cutting speed in certain embodiments, the pulse repetitionfrequency is selected so as to provide cutting speeds of conventionalsaw or laser semiconductor cutting processes. High pulse repetitionfrequencies are used to ablate material quicker. Further, high pulserepetition frequencies allow more energy to be used for ablation beforeit is dissipated in the surrounding materials.

As discussed in detail below, beam shaping is used in certainembodiments to improve kerf quality. Laser beams can be shaped tocreate, for example, a substantially flat kerf bottom that generatesless debris and reduces or eliminates damage to the substrate. Inaddition to an improved sidewall profile, beam shaping also reduces thewidth of the recast oxide layer.

For convenience, the term cutting may be used generically to includescribing (cutting that does not penetrate the full depth of a targetwork piece) and throughcutting, which includes slicing (often associatedwith wafer row separation) or dicing (often associated with partsingulation from wafer rows). Slicing and dicing may be usedinterchangeably in the context of this disclosure.

Reference is now made to the figures in which like reference numeralsrefer to like elements. For clarity, the first digit of a referencenumeral indicates the figure number in which the corresponding elementis first used. In the following description, numerous specific detailsare provided for a thorough understanding of the embodiments disclosedherein. However, those skilled in the art will recognize that theinvention can be practiced without one or more of the specific details,or with other methods, components, or materials. Further, in some cases,well-known structures, materials, or operations are not shown ordescribed in detail in order to avoid obscuring aspects of theinvention. Furthermore, the described features, structures, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

FIGS. 2A-2C are side view schematics of an exemplary work piece 200 thatis cut according to certain embodiments of the invention. The work piece200 includes layers 202, 204, 206 formed over a substrate 208. As anartisan will recognize, the layers 202, 204, 206 may includeinterconnect layers separated by insulation layers, including low-kdielectrics, to form electronic circuitry. For example, the layers 202,204, 206 may include materials such as Cu, Al, SiO₂, SiN, fluorsilicatedglass (FSG), organosilicated glass (OSG), SiOC, SiOCN, and othermaterials used in IC manufacture. For illustrative purposes, threelayers 202, 204, 206 are shown in FIGS. 2A-2C. However, an artisan willrecognize that more layers or fewer layers can be used for particularICs. As shown, the substrate 208 comprises Si. However, an artisan willalso recognize that other materials useful in IC manufacture can be usedfor the substrate 208 including, for example, glasses, polymers, metals,composites, and other materials. For example, the substrate 208 mayinclude FR4.

Electronic circuitry is formed in active device areas 210, 212 that areseparated from each other by scribing lanes or streets 214. An artisanwill recognize that test structures are often formed in and around thestreets 214. To create individual ICs, the work piece 200 is scribed,throughcut, or both, along the streets 214. In FIG. 2A, a laser beam 216according to one embodiment is shown scribing the work piece 200 byablating the layers 202, 204, 206 in the area of the street 214. Asshown in FIG. 2B, the result of the laser scribing process is a laserkerf 218 that passes from the top surface of the top layer 202 throughthe layers 202, 204, 206 to the top surface of the substrate 208. Asdiscussed below, in certain embodiments, the laser beam 216 is shaped soas to increase the quality of the kerf's sidewall profile and reduce orprevent damage to the substrate 208.

The laser beam 216 comprises a series of laser pulses configured toprovide the lowest possible fluence to the work piece 200 that stillprovides a desired material ablation of the layers 202, 204, 206 and/orthe substrate 208. In one embodiment, the fluence of the laser beam 216is selected so as to be in a range between the ablation threshold of thework piece 200 and approximately ten times the ablation threshold of thework piece 200. In another embodiment, the fluence of the laser beam 216is selected so as to be in a range between the ablation threshold of thework piece 200 and approximately five times the ablation threshold ofthe work piece 200.

To lower the ablation threshold, according to one embodiment, the pulsewidth is set in a range between approximately 0.1 picosecond andapproximately 1000 picoseconds. In other embodiments, the pulse width isset in a range between approximately 1 picosecond and approximately 10picoseconds. In other embodiments, the pulse width is set in a rangebetween 10 picoseconds and 40 picoseconds. However, an artisan willrecognize from the disclosure herein that other pulse widths can beused. For example, in one embodiment, the pulse width is in a rangebetween approximately 0.6 picosecond and approximately 190 picoseconds,while in another embodiment, the pulse width is in a range betweenapproximately 210 picoseconds and 1000 picoseconds.

In one embodiment, the laser beam 216 is generated using an averagepower in a range between approximately 10 W and approximately 50 W andan energy per pulse between approximately 1 μJ and approximately 100 μJ.When the layers 202, 204, 206 have a combined thickness in a rangebetween approximately 8 μm and approximately 12 μm, the laser beam 216is configured to cut through the layers 202, 204, 206 at a rate in arange between approximately 200 mm/second and approximately 1000mm/second using high pulse repetition frequencies.

In certain embodiments, the separation between pulses is in a rangebetween approximately 1 nanosecond and approximately 10 nanoseconds toallow substantially complete heat dissipation. In other embodiments, theseparation between pulses is in a range between approximately 10nanoseconds and approximately 1 microseconds to allow the plume ofablated material in a first pulse to spread to a sufficiently lowdensity so as to not significantly interact with a subsequent pulse. Incertain such embodiments, the pulse repetition frequency is in a rangebetween approximately 1 MHz and approximately 100 MHz. In otherembodiments, the pulse repetition frequency is in a range betweenapproximately 5.1 MHz and approximately 100 MHz. In another embodiment,the pulse repetition frequency is in a range between approximately 50kHz and approximately 4 MHz.

At high pulse repetition frequencies (e.g., above approximately 1 MHzand, more particularly, above approximately 10 MHz), residual pulseenergy may accumulate in the form of heat because deposited energy doesnot have sufficient time to dissipate between pulses. The cumulativeeffects generally increase ablation efficiency and may also increasemelting. However, the melting is generally limited to the irradiatedarea and may be concentrated in the center of the kerf. Depending on theparticular application, increased melting in the center of the kerf mayincrease or decrease the desired quality of the kerf.

In an exemplary embodiment, the laser beam 216 is generated using aDuetto™ laser available from Time-Bandwidth Products of Zurich,Switzerland. The Duetto™ laser has a wavelength of approximately 1064nm, a pulse repetition frequency in a range between approximately 50 kHzand approximately 4 MHz, an average power of approximately 10 W or more,a peak power of up to approximately 16 MW, an energy per pulse of up toapproximately 200 μJ, and a pulse width of up to approximately 12picoseconds. Alternatively, in another exemplary embodiment, the laserbeam 216 is generated using a “RAPID” picosecond laser available fromLumera-Laser GmbH of Kaiserslautern, Germany.

Harmonics of the 1064 nm laser can also be used to improve ablation forspecific materials. For example, a wavelength of approximately 532 nmcan be used to ablate Cu, a wavelength of approximately 355 nm can beused to ablate Si and certain low-k dielectrics, and a wavelength ofapproximately 266 nm can be used to ablate glass. In one embodiment, thewavelength is selected based at least in part on the respectivematerials and relative thicknesses of the layers 202, 204, 206 and/orthe substrate 208 so as to increase cutting speed. For example, thewavelength may be optimized to ablate a thick Cu layer rather than arelatively thin dielectric layer. In an alternative embodiment, thewavelength may be changed between ablation of one or more of the layers202, 204, 206, and/or the substrate 208. An artisan will also recognizethat using the harmonics will also improve the ability to focus thelaser beam because focusing is dependent on wavelength.

To scribe the layers 202, 204, 206, according to one embodiment, thefluence of each laser pulse is set at or above the highest ablationthreshold in the stack of layers 202, 204, 206 for a given wavelength,pulse energy, and pulse duration. In one embodiment, the fluence of eachlaser pulse is set in a range between approximately one and ten timesthe highest ablation threshold in the stack. In another embodiment, thefluence of each laser pulse is set in a range between approximately oneand five times the highest ablation threshold in the stack.

For example, it may be determined that the third layer 206 has a higherablation threshold than the first and second layers 202, 204. Thus,using short pulses in the picosecond range, setting the fluence of thelaser pulses so as to ablate the third layer 206 also provides ablationof the first and second layers 202, 204. In an exemplary embodiment, thefluence is set at approximately 1.5 times the highest ablation thresholdin the stack. If, for example, the third layer 206 has an ablationthreshold of approximately 10 J/cm² at a pulse width of approximately 10picoseconds, the laser beam 216 is configured to generate approximately20 μJ pulses with a spot size of approximately 10 μm to achieve afluence in a range between approximately 15 μJ/cm² and approximately 20μJ/cm².

An artisan will recognize that more layers or less layers may be ablatedor partially ablated during the laser scribing process. For example, thelaser beam 216 may be configured to ablate the top two layers 202, 204without ablating the third layer 206. Alternatively, as illustrated inFIG. 2C, the laser beam 214 may be configured to cut through the layers202, 204, 206 and the substrate 208 to fully separate the active deviceareas 210, 212 from one another (e.g., dicing). In certain embodiments,silicon substrates having a thickness in a range between approximately10 μm and approximately 760 μm are throughcut using a laser cuttingprocess. Artisans will recognize from the disclosure herein that othersubstrate thicknesses can also be throughcut according to the methodsdescribed herein.

However, as shown in FIGS. 2A and 2B, in one embodiment, the work piece200 is scribed to remove at least a portion of the layers 202, 204, 206in the street 214. The work piece 200 can then be mechanically broken ormechanically sawed along the kerf path 218 to complete the dicingprocess. Thus, materials that may be damaged by the saw and/or that candamage the saw, such as low-k dielectrics or test structures, can beremoved before sawing. In one embodiment, the saw follows the kerf 218so as to not touch the layers 202, 204, 206. Advantageously, crackingand debris are reduced, die break strength is increased and overallyield is improved.

FIG. 3A is a perspective view of a work piece 300 cut according toanother embodiment of the invention. The work piece 300 includes layers302, 304 formed over a substrate 306. As discussed above, the layers302, 304 may include, for example, materials such as Cu, Al, SiO₂, SiN,fluorsilicated glass (FSG), organosilicated glass (OSG), SiOC, SiOCN,and other materials used in IC manufacture. The substrate 306 mayinclude, for example, Si, FR4, glass, polymer, metal, compositematerial, and other materials used in IC manufacture.

FIG. 3B is a side view schematic of the work piece 300 shown in FIG. 3A.As shown, electronic circuitry is formed in active device areas 308, 310that are separated from each other by a street 312. In this example, thework piece 300 is scribed such that laser kerfs 314, 316 are formedusing laser parameters described herein on both sides of the street 312.In one embodiment, the laser kerfs 314, 316 are each in a range betweenapproximately 5 μm and approximately 10 μm wide. As shown in FIGS. 3Aand 3B, in certain embodiments, the laser kerfs 308, 310 extend into thesubstrate 306. However, in other embodiments, the laser kerfs 308, 310remove material only in one or both of the layers 302, 304.

The laser scribes 314, 316 act as “crack stops” or physical barriers forheat and mechanical stress during further processing. Thus, the laserscribes 314, 316 provide mechanical separation and thermal separationbetween the street 312 and the active device areas 308, 310. Forexample, after creating the laser scribe lines 314, 316 using laserablation techniques described herein, the street 312 can be mechanicallysawed to dice the active device areas 308, 310. The harsh effects ofsawing the street 312 do not propagate to the active device areas 308,310 such that cracking and chipping associated with mechanical sawing isreduced or eliminated in these areas.

As discussed above, in certain embodiments, the laser beam 216 shown inFIG. 2A is shaped so as to increase the quality of the kerf's sidewallprofile and reduce or prevent damage to the substrate 208. FIG. 4graphically illustrates the difference between a simplified Gaussianbeam irradiance profile 402 and a simplified shaped beam irradianceprofile 404. The center of the Gaussian beam irradiance profile 402 ismuch larger than the vaporization threshold 406 and the meltingthreshold 408 as compared to the shaped beam irradiance profile 404.Thus, the Gaussian beam puts a larger amount of excessive energy intothe target material, especially at the center of the beam.

Further, the slope of the Gaussian beam irradiance profile 402 betweenthe melting threshold 408 and the vaporization threshold is less thanthat of the shaped beam irradiance profile 404. Thus, the Gaussian beamwill produce a wider recast oxide layer because a wider area of materialwill be melted but not vaporized. For example, the arrows 410 representthe width of the recast oxide layer produced by the Gaussian beam whilethe arrows 412 represent the width of the recast oxide layer produced bythe shaped beam. Due to the rapid slope of the shaped beam irradianceprofile 404 between the melting threshold 408 and the vaporizationthreshold 406, the shaped beam produces a narrower recast oxide layer.

FIGS. 5A-5C graphically illustrate the difference between beam crosssection profiles. FIG. 5A shows a Gaussian cross section profile 510.FIGS. 5B-5C show “top hat” shaped cross section profiles. FIG. 5B showsa square cross section profile 512 and FIG. 5C shows a round crosssection profile 514.

U.S. Pat. Nos. 6,433,301 and 6,791,060 to Dunsky et al. disclose systemsand methods for beam shaping according to certain embodiments. FIGS.6A-6D show exemplary substantially uniform irradiance profiles producedby a Gaussian beam propagating through a diffractive optical element(DOE) as described in U.S. Pat. No. 5,864,430. FIGS. 6A-6D show “tophat” shaped beams. FIGS. 6A-6C show square irradiance profiles, and FIG.6D shows a cylindrical irradiance profile. The irradiance profile ofFIG. 6C is “inverted,” showing higher intensity at its edges than towardits center. Beam shaping components can be selected to produce pulseshaving an inverted irradiance profile shown in FIG. 6C that is clippedoutside dashed lines 610 to facilitate ablation to further improve kerftaper. Artisans will appreciate that beam shaping components can bedesigned to supply a variety of other irradiance profiles that might beuseful for specific applications.

FIG. 7 is an electron micrograph of a kerf 700 scribed throughinterconnect and low-k dielectric layers 702. The kerf 700 isapproximately 35 μm wide and was scribed using a laser having awavelength of approximately 355 nm. As described herein, a short pulsewidth (e.g., in the picosecond range) and a rapid pulse rate frequencywere used to achieve low fluence ablation at high speeds. The kerf 700was scribed at a speed of over 500 mm/s with a “top hat” shaped beam.The beam shaping provides a kerf bottom that is substantially flat andsides that are substantially vertical and well defined. Further, thereis substantially no chipping or cracking.

In certain embodiments disclosed herein, scribing can be accomplishedusing a single laser pass. However, in certain other embodiments, theamount of material removed per pulse is not sufficient to achieve thedesired scribe depth in one laser pass. In certain such embodiments,each location in the scribe line is exposed with multiple pulses toachieve desired material removal. In one such embodiment, material isexposed with pulses overlapping in the direction of a cut.

For example, FIG. 8 schematically illustrates sequentially exposing awork piece 800 to laser pulses in the direction of a cut. Each pulseablates a certain spot size 802 to a pulse ablation depth. To achieve anoverall ablation depth, sequential pulses have an overlap offset or bitesize 804 in the direction of the cut. For example, a first pulse removesmaterial in a first area 806. Then, a second pulse shifted in thedirection of the cut (e.g., to the left in FIG. 8) removes additionalmaterial from a second area 808 and a third area 808′. The width of thesecond area 808 and the third area 808′ (when combined) is the same asthe width of the first area 806 (e.g., the spot size 802). The secondarea 808 is to the side of the first area 806 in the direction of thecut and has a width equal to the bite size 804. The third area 808′ isbelow a portion of the first area 806. Thus, the overall ablation depthincreases from the first pulse to the second pulse.

The scribing process continues as additional pulses are sequentiallyapplied to the work piece 800 in the direction of the cut. The overalldepth of the cut increases with each pulse until a desired depth 810 isreached. After the desired depth 810 is reached, additional pulsescontinue to remove material in the direction of the cut withoutincreasing the overall depth past the desired depth 810. For a givenspot size 802, the bite size 804 will define the desired depth 810. Thedesired depth 810 is equal to the pulse ablation depth of a single pulsemultiplied by the ratio of the spot size 802 to the bite size 804. Forthe example shown in FIG. 8, the bite size 804 is approximatelyone-seventh the size of the spot size 802. Thus, the desired depth 810is seven times the pulse ablation depth of a single pulse (and is firstreached after seven pulses).

In one embodiment, the cut speed is controlled by first selecting thepulse duration as discussed above to reduce the threshold of ablation.Advantageously, in certain embodiments, a pulse width is selected thatprovides substantially the lowest ablation threshold for the targetmaterial or materials. Then, the spot size is selected to provide thedesired fluence for a selected energy per pulse. Based on the singlepulse ablation depth, the bite size is then selected to provide theoverall ablation depth. As discussed above, the pulse repetitionfrequency is then selected to increase cut speed. In certainembodiments, lower pulse repetition frequencies (e.g., approximately 70kHz) are used with high pulse energies (e.g., approximately 50 μJ toapproximately 100 μJ) to ablate at a low fluence and a high speed bychanging the aspect ratio of the laser spot from symmetrical (e.g.,circular) to asymmetrical (e.g., elliptical or rectangular) so as todistribute the energy of the pulses in the direction of the cut. Thus,rather than focusing the energy on a small circular spot, an ellipticalor rectangular spot spreads the energy of each pulse to lower thefluence while removing material in the direction of the cut. A shapedrectangular beam, for example, may be configured such that the longerdimension of the rectangle is in the direction of the cut.

Although the embodiments above have been described with respect tosingulating semiconductor wafers, artisans will recognize otherapplications such as memory repair and laser micromachining. Forexample, FIG. 9 is an electron micrograph of a micromachined pattern 900in a semiconductor material using the laser ablation methods describedabove. The exemplary pattern 900 includes trenches 902 approximately 51μm wide cut in a precise pattern. The trenches 902 have substantiallyflat bottoms and well defined side walls. Further, the distance 904between trenches is as small as approximately 25 μm.

Artisans will recognize from the disclosure herein that other patternsand more precise cuts can also be achieved. For example, FIG. 10 is anelectron micrograph of a micromachined pattern 1000 in a semiconductormaterial using the laser ablation methods described above. The exemplarypattern 1000 includes trenches 1002 approximately 50 μm wide that areseparated in some locations 1004 by distances approximately 10 μm wide.

In FIGS. 7, 8 and 9, it can be observed that there is substantially nochipping, cracking or contamination. In certain embodiments, somecleaning may be desired to remove small amounts of debris. For example,conventional high pressure water or solid CO₂ “sand blasting” techniquescan be used after laser ablation to remove particles or debris. However,the ablation processes discussed herein are generally cleaner thanconventional laser or mechanical saw cutting techniques and require lesscleaning than conventional processes or no cleaning at all. Thus,additional lateral separation between devices on a wafer is not requiredto accommodate the dicing process. Further, due to the low fluence usedwith short wavelengths, there are less problems with heat affectedzones, cracking, peeling, and chipping. Thus, higher die break strengthsand overall process yields are achieved.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

What is claimed is:
 1. A method of cutting a work piece including one ormore layers formed over a substrate, the method comprising: generating abeam of laser pulses; shaping the generated beam of laser pulses throughdiffraction to produce spatial irradiance profiles with flattened tops,wherein the shaping produces, at the work piece, a series of laser spotsthat each include an asymmetrical aspect ratio with a first dimensionbeing longer than a second dimension, the shaping spatially spreadingthe energy of each of the laser pulses as the aspect ratios of therespective laser spots change from symmetrical to asymmetrical; aligningthe longer first dimension of each laser spot in a cutting directionalong the work piece so as to distribute the energy of each laser pulsein the cutting direction; and providing relative movement, in thecutting direction, between the beam and the work piece so as to scribe akerf in the one or more layers, the kerf passing through the one or morelayers to a top surface of the substrate.
 2. The method of claim 1,wherein the laser pulses have a pulse width in a range betweenapproximately 0.1 picosecond and approximately 1000 picoseconds.
 3. Themethod of claim 1, wherein the beam has a pulse repetition rate in arange between approximately 100 kHz and approximately 100 MHz.
 4. Themethod of claim 1, wherein the energy per pulse is in a range betweenapproximately 1 μJ and approximately 100 μJ.
 5. The method of claim 1,wherein the average power of the beam is approximately 10 Watts or more.6. The method of claim 1, wherein the one or more layers have a combinedthickness in a range between approximately 8 μm and approximately 12 μm,and wherein the beam is configured to scribe the kerf through the one ormore layers at a rate in a range between approximately 200 mm/second and1000 mm/second.
 7. The method of claim 1, further comprising cutting thesubstrate with a saw along the length of the kerf.
 8. The method ofclaim 1, wherein the kerf forms a first scribe line and a second scribeline separating a first active device area from a second active devicearea.
 9. The method of claim 8, further comprising cutting the one ormore layers and the substrate with a saw between the first scribe lineand the second scribe line.
 10. The method of claim 1, furthercomprising cutting with the beam through the one or more layers and thesubstrate in a single pass.
 11. The method of claim 1, furthercomprising cutting with the beam through the one or more layers and thesubstrate in a plurality of passes.
 12. The method of claim 1, whereinat least one of the one or more layers comprises a low-k dielectricmaterial.
 13. The method of claim 1, wherein the beam has a fluence in arange between the selected laser ablation threshold and approximately 5times the selected laser ablation threshold.
 14. A method of scribing awafer having a plurality of integrated circuits formed thereon ortherein, the integrated circuits separated by one or more streets, themethod comprising: generating a beam of laser pulses, the laser pulseshaving a pulse width selected so as to minimize an ablation threshold ofa target material; shaping the generated beam of laser pulses throughdiffraction to convert the laser pulses into laser pulses with spatialirradiance profiles having flattened tops, wherein the shaping produces,at the wafer, a series of laser spots that each include an asymmetricalaspect ratio with a first dimension being longer than a seconddimension, the shaping spatially spreading the energy of each of thelaser pulses as the aspect ratios of the respective laser spots changefrom symmetrical to asymmetrical; aligning the longer first dimension ofeach laser spot in a cutting direction along the wafer so as todistribute the energy of each laser pulse in the cutting direction; andproviding relative movement, in the cutting direction, between the beamand the wafer so as to ablate a portion of the target material with thebeam at a pulse repetition frequency in a range between approximately5.1 MHz and approximately 100 MHz.
 15. The method of claim 14, whereinthe pulse width is in a range between approximately 0.1 picosecond andapproximately 1000 picoseconds.
 16. The method of claim 14, wherein theenergy per pulse is in a range between approximately 1 μJ andapproximately 100 μJ.
 17. The method of claim 14, wherein the targetmaterial has a thickness in a range between approximately 8 μm andapproximately 12 μm, and wherein the beam is configured to cut throughthe target material at a rate in a range between approximately 200mm/second and 1000 mm/second.
 18. A method of scribing a wafer having aplurality of integrated circuits formed thereon or therein, theintegrated circuits separated by one or more streets, the methodcomprising: generating a beam of one or more laser pulses, the laserpulses each having a pulse width in a range between approximately 0.6picosecond and approximately 190 picoseconds; shaping the generated beamof laser pulses through diffraction to convert the laser pulses intolaser pulses with spatial irradiance profiles with flattened tops,wherein the shaping produces, at the wafer, a series of laser spots thateach include an asymmetrical aspect ratio with a first dimension beinglonger than a second dimension, the shaping spatially spreading theenergy of each of the laser pulses as the aspect ratios of therespective laser spots change from symmetrical to asymmetrical; aligningthe longer first dimension of each laser spot in a cutting directionalong the wafer so as to distribute the energy of each laser pulse inthe cutting direction; and providing relative movement, in the cuttingdirection, between the beam and the wafer so as to ablate a portion ofthe target material with the beam.
 19. The method of claim 18, whereinthe beam has a pulse repetition frequency in a range betweenapproximately 100 kHz and approximately 100 MHz.
 20. A method ofscribing a wafer, the method comprising: generating a beam of one ormore laser pulses, the laser pulses having a pulse width in a rangebetween approximately 210 picoseconds and approximately 1000picoseconds; and providing relative movement, in a cutting direction,between the beam and the wafer so as to ablate a portion of targetmaterial with the beam, the relative movement including a bite sizecomprising an overlap offset between successive laser spots at thewafer, the bite size based on an overall depth of the ablated portionsuch that the overall depth of the ablated portion equals a pulseablation depth of a single laser pulse multiplied by a ratio of thefirst dimension to the bite size.
 21. The method of claim 20, whereinthe beam has a pulse repetition frequency in a range betweenapproximately 100 kHz and approximately 100 MHz.